Schematic illustration of notothenioid craniofacial evolution via heterochrony. Panel A summarizes results for the relative patterns of each collagen gene expression during notothenioid larval skeletogenesis. Expression in G. aculeatus and D. rerio follows a typical vertebrate pattern, with col2a1 expressed first in differentiating chondrocytes, followed by a down regulation of col2a1 and subsequent up regulation of the bone markers, col10a1 and col1a1 (A, top panel). The notothenioid pattern is quite different, with sustained high levels of col2a1 expression throughout later periods of larval development, expression of col10a1 limited to a small subset of elements, and weak col1a1 expression throughout the pharyngeal skeleton (A, bottom panel). The x-axis denotes developmental time starting just before chondrocyte differentiation, and the y-axis represents relative (not quantitative) levels of collagen expression. Panels B and C depict a model for anterior-posterior and dorsal-ventral patterning of the pharyngeal skeleton. Compared to stickleback and zebrafish, the developmental program in notothenioid species is shifted toward the ventral components (darkly shaded) of the anterior-most craniofacial elements (purple and blue). This shift results in the development of rostral-caudally expanded Meckel's (purple) and ceratohyal (blue) cartilages, and reduced posterior cartilage elements (i.e., fifth ceratobranchial cartilage, yellow).